Radio-frequency loss reduction in photonic circuits

ABSTRACT

In photonic integrated circuits implemented in silicon-on-insulator substrates, non-conductive channels formed, in accordance with various embodiments, in the silicon device layer and/or the silicon handle of the substrate in regions underneath radio-frequency transmission lines of photonic devices can provide breaks in parasitic conductive layers of the substrate, thereby reducing radio-frequency substrate losses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalApplication No. 62/148,353, filed on Apr. 16, 2015, which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The instant disclosure relates generally to semiconductor-wafer-bondedphotonic structures, and more particularly to photonic integratedcircuits and electrical radio-frequency (RF) losses therein.

BACKGROUND

Photonic integrated circuits (PICs) generally integrate multiplephotonic devices and respective photonic functions in the same chip,e.g., within the device layer(s) of a silicon on insulator (SOI)substrate with bonded III-V material. One or more of the photonicdevices may include metal transmission lines disposed on top of thesubstrate. These transmission lines, which generally operate in theradio-frequency (RF) regime, can in principle carry large amounts ofdata at high speeds, data transmission being subject, however, topropagation loss due, at least in part, to substrate losses.

FIG. 1 depicts, in cross-sectional view, a portion of an SOI substrate100 with an RF transmission line 102 disposed thereon, illustrating thebasic layer structure of the substrate 100 and the origin of RF lossestherein. As shown, the SOI substrate 100 includes a silicon handle 104,a buried oxide (BOX) layer 106 (e.g., made substantially of silicondioxide) thereabove, a silicon device layer 108 on top of the BOX layer106, and, optionally, a cladding layer 110 (e.g., made of a dielectricmaterial) above the silicon device layer 108. (In the absence of aseparate cladding layer 110, air serves as the cladding.) Photonicdevices (not shown) may be embedded in the silicon device layer 108and/or the cladding layer 110. The transmission line 102 generallyincludes multiple metal traces (e.g., two, as shown) that function aselectrodes. As an RF electrical current is applied to these electrodes,an RF electrical field that extends through the SOI substrate 100 isformed, as indicated by the field lines 112. Parasitic conductive layers114 formed at the interfaces of the BOX layer 106 with the siliconhandle 104 and with the silicon device layer 108 allow for electriccurrents to flow in response to the electric field, reducing its fieldstrength and thereby contributing to the RF losses in the substrate 100;substantial losses may occur, for example, in the frequency range fromabout 0.1 GHz to 40 GHz or even higher. In order to enable lowerpropagation loss in the transmission line 102, it is desirable to findways of reducing these substrate losses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a portion of an SOI substrate withan RF transmission line disposed thereon, illustrating the basic layerstructure of the substrate and the origin of RF losses therein.

FIG. 2 is a top view of a portion of an example PIC comprising anexample Mach-Zehnder modulator with two electrodes, in accordance withone embodiment.

FIGS. 3A-3C are cross-sectional views of the example Mach-Zehndermodulator of FIG. 2 as implemented on an SOI substrate having a channelformed in the silicon handle, in accordance with one embodiment.

FIG. 3D is a cross-sectional view of the example Mach-Zehnder modulatorof FIG. 2 as implemented on an SOI substrate having a filled channelformed in the silicon handle, in accordance with one embodiment.

FIG. 3E is a cross-sectional view of the portion of the example PIC ofFIG. 2 as implemented on an SOI substrate having a channel formed in thesilicon handle, in accordance with one embodiment, illustrating thelateral confinement of the channel to a region underneath a singlephotonic device.

FIG. 4A is a top view of a Mach-Zehnder modulator with three electrodes,in accordance with one embodiment.

FIG. 4B is a cross-sectional view of the example Mach-Zehnder modulatorof FIG. 4A as implemented on an SOI substrate having a channel formed inthe silicon handle, in accordance with one embodiment.

FIGS. 4C and 4D are top views of example Mach-Zehnder modulators withfour and five electrodes, respectively, in accordance with variousembodiments.

FIG. 5 is a top view of an example photodetector with a waveguide andincluding electrodes, in accordance with various embodiments.

FIG. 6 is a flow chart of an example method of fabricating an SOIsubstrate having one or more channels formed therein, in accordance withvarious embodiments.

DESCRIPTION

Described herein are PICs implemented in SOI substrates in which one ormore nonconductive channels in the silicon device layer and/or thesilicon handle provide breaks in the parasitic conductive layers, aswell as methods for manufacturing such PICs. In accordance with variousembodiments, the channels are formed in regions underneath atransmission line, each channel laterally at least partially overlappingwith a gap defined between two adjacent electrodes of the transmissionline so as to prevent lateral current flow between the electrodes. Thechannels do not extend laterally beyond the outer edges of therespective electrodes defining the gap, and each channel is, thus,confined to a region encompassing only a single one of generallymultiple photonic devices included in the PIC. (The terms “outer edge”and “inner edge” are used with reference to the gap between two adjacentelectrodes. The inner edge of an electrode defines one of the boundariesof the gap, and the outer edge is the other edge of that electrode,which is more distant from the gap. The term “lateral” herein denotes adirection generally perpendicular to the direction of the transmissionline and its electrodes in a plane parallel to the layers of the SOIsubstrate.) Compared with some conventional approaches for RF lossreduction, which involve either the removal of a large portion of thesilicon handle spanning multiple photonic devices and leaving, in someinstances, only a narrow rim of the handle, or the etching of channelsextending through the cladding, silicon device layers, and BOX layers ofthe SOI substrate, the present approach involves the removal of muchsmaller amounts of material, resulting in better mechanical durabilityand ease of handling of the PIC while still achieving the purpose ofbreaking the parasitic conduction layer so as to reduce RF losses. Insome embodiments, the channels are wholly within the region directlyunderneath the gap and do not overlap laterally with the electrodesdefining the gap; in some embodiments, the channels are even narrower inwidth than the gap and do not even extend to the inner edges of theelectrodes. The gaps, and thus the channels if no wider than the gaps,may have widths between about 5 μm and about 100 μm. PICs with suchnarrow channels can be made, e.g., by minimally etching the uppersilicon device layer and/or etching the silicon handle using back-sidetrenches or back-side vias. The etched channels may be filled with anonconductive polymer or dielectric material, or left air-filled.

In accordance with various embodiments, a photonic integrated circuit(PIC) may include a silicon-on-insulator substrate including a siliconhandle, a buried oxide layer disposed on top of the silicon handle, asilicon device layer disposed on top of the buried oxide layer, and acladding layer disposed on top of the silicon device layer, and, formedpartially within the silicon device layer and/or the cladding layer, aplurality of photonic devices. (A device is herein understood to be“formed partially within” a layer if one or more of the structures ofthe device are embedded in the layer and/or created by removal ofmaterial from the layer. For example, a device structure may be etchedinto the silicon device layer, or created from III-V material embeddedwithin the cladding layer.) One of the photonic devices includes atransmission line having two or more electrodes disposed on top of thecladding layer, the two or more electrodes defining one or more gapsbetween adjacent ones of the electrodes. One or more nonconductivechannels each laterally at least partially overlapping with one of theone or more gaps and not extending beyond outer edges of the electrodesdefining the respective gap are formed within the silicon handle and/orthe silicon device layer. (The device including the transmission lineand having one or more nonconductive channels laterally overlapping withthe gap(s) between the electrodes of the transmission line is hereinalso referred to as the “first photonic device,” to distinguish it fromone or more other devices contained in the same PIC, referred to as “oneor more second photonic devices,” which may, but need not, includetransmission lines and, even if they include transmission lines, neednot have nonconductive channels formed underneath. In general, if a PICincludes multiple photonic devices with RF transmission lines, RF lossesmay be reduced in any one or more of them by forming nonconductivechannels in the silicon handle and/or the silicon device layer asdescribed herein.)

Various example embodiments will now be described in more detail withreference to the accompanying drawings. Numerous specific features anddetails mentioned in the following description do not apply to, and notto be read into, each and every embodiment of the disclosed subjectmatter, but merely serve the purpose of better illustration andexplanation.

With reference to FIGS. 2 and 3A-3E, the principles of the presentlydisclosed subject matter are illustrated for an example PIC including aMach-Zehnder modulator, among other photonic devices. A Mach-Zehndermodulator is an optical device (usually referred to as a “photonicdevice” if implemented in semiconductor) that serves to control theamplitude of an optical signal via an applied electrical signal. TheMach-Zehnder modulator generally includes an optical waveguide branchingout into two interferometric waveguide arms that re-combine at the otherend, and a transmission line with at least two electrodes to create anelectrical field across the waveguide arms that imparts a relative phaseshift between optical signals travelling through the two waveguide arms.

FIG. 2 is a top view of a portion of an example PIC including an exampleMach-Zehnder modulator 200 with two electrodes, 202, 203, in accordancewith one embodiment. To illustrate that the Mach-Zehnder modulator 200is only one among multiple photonic devices within the PIC, the depictedportion also shows a second device 205, which includes a waveguide andtwo electrodes.

Returning to the description of the Mach-Zehnder modulator 200, the twoelectrodes 202, 203 carry opposite polarity of the otherwise sameelectrical signal so as to create an electrical field extending from oneelectrode 203 to the other electrode 202. In the gap 210 formed betweenthe two electrodes 202, 203, an incoming optical waveguide 212 splitsinto two interferometric waveguide arms 214, 215, which are thenrecombined into an outgoing waveguide 217. (While shown as running alongthe entire length of the waveguide arms 214, 215 and including thepoints of splitting and recombination, the electrodes 202, 203 may,alternatively, be shorter so as to extend along only a portion of thewaveguide arms 214, 215.) The waveguide arms 214, 215 may be formed, atleast partially, in a different layer of the SOI substrate than theincoming and outgoing waveguides 212, 217, as indicated throughdifferent shadings and further illustrated in FIG. 3B. For example, thewaveguide arms 214, 215 may include III-V structures bonded to the SOIsubstrate and embedded within the cladding layer 110, which achieve aphase shift between the waveguide arms 214, 215. Each waveguide arm 214,215 is electrically connected to the adjacent electrode 202, 203 via oneor more (e.g., four, as shown) electrical contacts 220. As shown, theelectrical contacts 220 may be T-shaped, with a narrow bridge extendingfrom the electrode 202 or 203 to the respective waveguide arm 214 or 215and a contact trace disposed directly on top of and running along aportion of the waveguide arm 214 or 215. Note that, for purposes ofclearly describing the relative position and extent of the channelsformed in the SOI substrate and the electrodes of the RF transmissionline, the electrical contacts 220 are deemed not part of the electrodes202, 203.

FIGS. 3A-3C are cross-sectional views of the example Mach-Zehndermodulator of FIG. 2, taken through the SOI substrate in which theMach-Zehnder modulator is implemented along three lines marked [A-A],[B-B], and [C-C] in FIG. 2. The first cross-section, along line [A-A],is taken at a position along the electrodes 202, 203 where it does notintersect the waveguide structure (in particular, waveguide arms 214 or215) between the electrodes 202, 203. In the depicted embodiment, as canbe seen in FIG. 3A, silicon has been removed from both the siliconhandle 104 and the silicon device layer 108 in a region directlyunderneath and laterally coextensive with the gap 210 between theelectrodes 202, 203 to form channels 300, 302 within the silicon handle104 and silicon device layer 108, respectively (the channels extendinginto the plane of FIG. 3A). The channel 302 in the silicon device layer108 is filled with the cladding material (as a result of themanufacturing process in accordance with one embodiment). The channel300 may remain unfilled (that is, air-filled).

FIG. 3B shows the cross-section along [B-B] through the interferometricwaveguide arms 214, 215. In this embodiment, the waveguide arms 214, 215are formed largely within the cladding layer 110, as III-V waveguides310, 312 extending from an embedded layer 313 of III-V semiconductormaterial. The waveguide arms 214, 215 may also be implemented in part inthe silicon device layer 110. For example, as shown, the silicon devicelayer 110 may be patterned to form channels underneath the III-Vwaveguides 310, 312 that define therein silicon rib waveguide cores 314,316 that appear here as an upside down “T” shape. One or more channels318, 320 formed in the silicon device layer 110 underneath the gapbetween the III-V waveguides 310, 312 as an extension of channel 300 addto the breaking of the conductive layers. The spatially periodic removalof material from the silicon device layer 110, results in multiple(e.g., two, as shown) channels 318, 320 may serve to achieve a suitabletrade-off between providing structural support for the III-V materialand minimizing charge build-up and RF losses. The cross-section in FIG.3B also shows the electrical contact traces (of electrical contacts 220)disposed on top of the III-V waveguides 310, 312, through which thewaveguide 310, 312 are connected to the electrodes 202, 203. FIG. 3Cfurther shows, in a cross-section along [C-C], the contact bridges ofthe electrical contacts 220 that connect the electrodes 212, 213 to thecontact traces.

FIG. 3D is a cross-sectional view of the example Mach-Zehnder modulatorof FIG. 2 in accordance with an alternative embodiment, in which thechannel 320 formed in the silicon handle 104 is filled with anon-conductive material, such as a non-conductive polymer (e.g., withoutlimitation, polyimide or benzocyclobutene) or a dielectric material(e.g., without limitation, spin-on-glass, SiO₂, SiNx, or Al₂O₃).

FIG. 3E is a cross-sectional view, taken along line [C-C], of the entireexample PIC portion shown in FIG. 2. In addition to the cross-sectionalportion through the Mach-Zehnder modulator (shown in FIG. 3C), FIG. 3Ealso includes a cross-section through the second device 205. In thisexample embodiment, silicon is removed in the silicon handle 104 only inthe region underneath the Mach-Zehnder modulator 200 to form channel300, but not in the region underneath the second device 205 or in aregion between the two devices 200, 205. By limiting, in this manner,silicon removal to one or more channels confined within one or moreindividual devices subject to RF losses, overall silicon removal fromthe silicon handle 104 is minimized and the structural support providedby the silicon handle 104 is maintained to the maximum extent whilereducing the RF losses.

Non-conductive channels formed in the silicon handle 104 or a silicondevice layer 108 underneath transmission lines (including electrodes andgaps between the electrodes) can reduce RF losses in many differentstructural implementations of Mach-Zehnder modulators as well as invarious other types of photonic devices, the structure depicted in FIGS.2 and 3A-3D being just one example. In general, channels breakingparasitic conductive layers, as described herein, may be particularlyuseful in long (>0.1 mm in length) RF photonic devices implemented inSOI substrates (including, but not limited to, SOI substrates withbonded III-V components).

Further, the possible number of electrodes in a single transmission lineis not limited to only two. FIGS. 4A, 4C and 4D, for instance, providetop views of example Mach-Zehnder modulators with three, four, and fiveelectrodes, respectively. The gaps between the electrodes may be betweenabout 1 μm and about 100 μm in width, and the electrodes themselves mayhave widths between about 2 μm and about 100 μm. In some examples, theimpedance of the transmission line formed with the electrodes can matchan external RF characteristic impedance between about 25 ohms and about100 ohms, and most commonly 50 ohms or 75 ohms. Devices with moreelectrodes tend to be (but are not necessarily) wider overall.

In the embodiment of FIG. 4A, the transmission line includes threeelectrodes: a central signal electrode 400 flanked by two groundelectrodes 402, 404. FIG. 4B is a cross-sectional view of the exampleMach-Zehnder modulator of FIG. 4A as implemented on an SOI substratehaving a channel formed in the silicon handle, in accordance with oneembodiment. As shown, the III-V layer 313 is, in this embodiment, brokenbetween the left and right interferometric waveguide arms 406, 408. Ascan be seen in FIG. 4A in conjunction with FIG. 4B, the signal electrode400 is disposed above the waveguide structure and electrically connectedto the top portion of the left interferometric waveguide arm 406 and thebottom portion of the right interferometric arm 408. The right groundelectrode 404 is connected to the top portion of the rightinterferometric waveguide arm 408 and the left ground electrode 402 isconnected to the bottom portion of the left interferometric arm 406. Toavoid RF losses in the depicted device, channels may be formedunderneath the gaps between the signal electrode 400 and each of groundelectrodes 402, 404.

In the embodiment of FIG. 4B, a Mach-Zehnder modulator with atransmission line having four electrodes is shown. In the center of thetransmission line, two signal electrodes 410, 412 of opposite polarityare disposed above and electrically connected to the two interferometricwaveguide arms 214, 215. The signal electrodes 410, 412 are flanked bytwo ground electrodes 414, 416. Here, channels may be formed in thesilicon handle and/or silicon device layer underneath the gap betweenthe two signal electrodes 410, 412 or the gaps between each signalelectrode and its adjacent ground electrode (e.g., between signalelectrode 410 and ground electrode 414 and/or between signal electrode412 and ground electrode 416).

In the embodiment of FIG. 4C, a Mach-Zehnder modulator with atransmission line having five electrodes is shown. Compared with thetransmission line of FIG. 4D, an additional ground electrode 420 isadded between the two signal electrodes 422, 424 of opposite polarity.The five electrodes 426, 422, 420, 424, 428 define four gaps betweenadjacent electrodes. More particularly, a gap is defined between thesignal electrode 422 and each of its adjacent ground electrodes 426, 420and between the signal electrode 424 and each of its adjacent groundelectrodes 420, 428. Channels that break the parasitic conductive layersmay be formed in the region under each of the gaps (possibly partiallyoverlapping with the electrodes defining the gap).

As an example of a different type of photonic device that may benefitfrom RF loss reduction in accordance herewith, FIG. 5 shows a top viewof an example traveling-wave photodetector including an opticalwaveguide 500 and two electrodes 502, 504 of opposite polarity (asdescribed, e.g., in U.S. Pat. No. 5,270,532 and U.S. Patent ApplicationPublication No. 2010/0327382). This device is used for photodetectionvia an RF signal that is generated on the electrodes by the absorbedlight. A single waveguide brings light into a long photoabsorptionregion (e.g., between about 0.1 mm and 10 mm in length) where the lightis gradually absorbed and an RF signal is generated on the electrodes.To preserve the RF signal, it is beneficial to reduce the opticallosses. The waveguide 500, which may be a III-V structure bonded to anSOI substrate, receives optical inputs from a waveguide 506 in thesilicon device layer. An electrical signal is generated as the opticalsignal is absorbed. To reduce RF losses in this device, one or morechannels may be formed in the silicon device layer 108 or silicon handle104 in a region laterally overlapping with the gap between theelectrodes 502, 504.

FIG. 6 is a flow chart of an example method 600 of fabricating an SOIsubstrate having one or more channels formed therein, in accordance withvarious embodiments. Throughout the method 600, variousphotolithographic patterning and etching operations are performed usingconventional techniques well-known to those of ordinary skill in theart. The method begins, at 602, with providing an SOI wafer, whichincludes a silicon handle 104, BOX layer 106, and silicon device layer108 (as yet without any devices) as depicted, e.g., in FIG. 1. Inoperation 604, the silicon device layer 108 is patterned and etched, inone or more iterations, to form photonic device components (e.g.,optical waveguides) and/or conductivity-breaking channels. For example,to create the structure shown in FIG. 3B, the silicon device layer 108is first patterned and partially etched to form the upper portion of therib waveguide 314, which looks like an upside down “T”, and thenpatterned again and etched down to the box layer 106 to form the lowerportion of the rib waveguide as well as any device components (such as,e.g., waveguide cores 314, 316) and conductivity-breaking channels(e.g., channels 302, 318, 320) therein. Next, a dielectric material isdeposited on top of the etched silicon device layer 108 to fill in theetched regions, and the surface is planarized (operation 606). In someembodiments, in operation 608, III-V material is bonded to the patternedSOI substrate and thinned to reduce the III-V material thickness afterbonding. To create the III-V device components (e.g., with reference toFIG. 3B, the III-V waveguides 310, 312 that form the interferometricwaveguide arms of the Mach-Zehnder modulator), the bonded III-V materialis patterned and etched (operation 610). Further, to create theelectrodes of the transmission lines (e.g., with reference to FIG. 2,electrodes 202, 203) and associated electrical contacts (e.g., contacts220) and thin and thick dielectrics (e.g., of cladding layer 110),dielectric materials and electrode metals are then deposited, patterned,and etched in a series of steps (operation 612). To formconductivity-breaking channels in the silicon handle 104, the patternedwafer is flipped and aligned (e.g., by through-wafer-alignment, whereinalignment features on the top surface are used to position aphotolithography mask precisely relative to the wafer backside), andthen back-side trenches are patterned and etched into the silicon handle104 (operation 614). Optionally, the back-side trenches may be filledwith a non-conductive polymer or dielectric (operation 616).

Although embodiments have been described with reference to specificexample embodiments, it will be evident that various modifications andchanges may be made to these embodiments without departing from thebroader spirit and scope of the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense. The accompanying drawings that form a parthereof, show by way of illustration, and not of limitation, specificembodiments in which the subject matter may be practiced. Theembodiments illustrated are described in sufficient detail to enablethose skilled in the art to practice the teachings disclosed herein.Other embodiments may be used and derived therefrom, such thatstructural and logical substitutions and changes may be made withoutdeparting from the scope of this disclosure. This description,therefore, is not to be taken in a limiting sense, and the scope ofvarious embodiments is defined only by the appended claims, along withthe full range of equivalents to which such claims are entitled.

The invention claimed is:
 1. A photonic integrated circuit (PIC)comprising: a silicon-on-insulator substrate comprising a siliconhandle, a buried oxide layer disposed on top of the silicon handle, asilicon device layer disposed on top of the buried oxide layer, and acladding layer disposed on top of the silicon device layer; a firstphotonic device comprising a device structure made of a materialembedded within the cladding layer and a transmission line having two ormore electrodes disposed on top of the cladding layer, the two or moreelectrodes defining one or more gaps between the electrodes; one or moresecond photonic devices formed partially within at least one of thesilicon device layer or the cladding layer; and formed within at leastone of the silicon handle or the silicon device layer in a regionunderneath the transmission line, one or more nonconductive channelseach laterally overlapping with one of the one or more gaps and notextending beyond outer edges of the electrodes defining the respectivegap.
 2. The PIC of claim 1, wherein the first photonic device is aMach-Zehnder modulator comprising two waveguide interferometer armsformed of the material.
 3. The PIC of claim 1, wherein the one or morechannels underneath the one or more gaps do not laterally overlap withthe electrodes defining the respective gaps.
 4. The PIC of claim 1,wherein at least one of the nonconductive channels is formed within thesilicon device layer.
 5. The PIC of claim 1, wherein at least one of thenonconductive channels is formed within the silicon handle.